1. Field of the Invention
The invention relates to method of preparing molded silicone rubber products using liquid silicone rubber (LSR). In particular, the invention uses a new liquid silicone rubber composition and process to produce cured silicone rubber products faster, with less equipment and less product variability.
2. Description of Related Art
Processes using liquid silicone rubbers (LSR) to form molded silicone rubber products have been around for almost four decades. Liquid silicone rubbers belong to the group of heat-curing rubbers. A characteristic feature is their low viscosity during processing compared to solid silicones or elastomers. Two-component mixtures increasingly crosslink by the addition process. This means that the reaction takes place without any decomposition product forming. This is an important benefit for the injection molding field since there are no byproducts of cure, there is no concern of deposits forming on the molds.
Typically, a two part platinum catalyzed addition cure reaction is used to make LSR molded rubber products, wherein the first component is a mixture of vinylsiloxane polymers, treated amorphous fumed silica, and platinum catalyst (component A) and the second component is a mixture of vinylsiloxane polymers, treated amorphous fumed silica, hydrogen siloxane crosslinking polymers, and a cure rate inhibitor (component B).
The A and B components are separately pumped and metered through a static mixer. The A and B mixture is further mixed in the LSR machine transferring screw prior to injection into the mold. The A and B mixture is then heat cured at a specific time and temperature depending on the part size. The finished cured product is automatically ejected from the mold, and the process repeated.
U.S. Pat. No. 3,884,866 discloses a two part LSR process using two different vinylsiloxane polymers, a platinum catalyst, and pre-treated silica filler for the first component, and the same vinylsiloxane polymers and pretreated silica filler plus a hydrogen containing polysiloxane and cure rate inhibitor as the second component. U.S. Pat. No. 4,162,243 discloses a two part LSR process using an in situ treated amorphous silica filler. U.S. Pat. No. 5,977,220 discloses a two part LSR process using a nitrogen organic cation salt to improve the compression set of the silicone mixture. U.S. Pat. No. 6,034,199 discloses a two part LSR process with improved cure rate inhibitors. U.S. Pat. No. 6,464,923 discloses a three part LSR process. The first component is a diorganopolysiloxane polymer and inorganic filler; the second component is a liquid catalyst and diorganopolysiloxane polymer mixture; and the third component is hydrogen siloxane mixed with an organopolysiloxane polymer. The patent also discloses the use of carbon black as an inorganic filler. The three separate parts result in improved storage stability over a two part LSR process.